24 April 2019–Researchers at the SSA 2019 Annual Meeting are discussing proposed revisions to the National Seismic Hazard Model (NSHM). The draft report concludes that there could be significantly increased ground shaking across many locations in the central and eastern United States, as well as four urban areas built on large sedimentary basins.

The U.S. Geological Survey’s NSHM is used to create maps that show the likelihood of a certain strength of damaging ground shaking in a particular area over a period of 50 years. The maps are used in determining seismic-compliant building codes, insurance rates and other public policies such as emergency response plans.

At the SSA meeting, USGS researcher Peter Powers will present the latest draft of the NSHM, now open for public comment, and explain the major updates to the model. Powers said some key additions to the model include an update of ground motion models for the central and eastern U.S., inclusion of new earthquakes that occurred between 2012 and 2017, and consideration of how ground motion may be amplified in the deep sedimentary basins underlying Los Angeles, San Francisco, Seattle and Salt Lake City.

“Due to population growth, more people live and work in areas of high or moderate seismic hazard than ever before, leading to higher risk of undesirable consequences from future ground shaking,” Powers and his colleagues write.

The NSHM draft report concludes that as many as 34 million people in the U.S. (about one in nine people) are expected to experience a strong level of shaking at least once in their lifetimes. Among other findings, the report notes that 32.2 million people under the 2018 model will be exposed to ground shaking intensities equivalent to VIII — the “severe” level causing considerable damage and potentially partial collapse in ordinary buildings — on the Modified Mercalli Intensity scale. This is a significant increase from 28.0 million under the 2014 model.